Analysis for the Cipherext Characteristic of Based on Search Mechanism Chaotic Cryptosystem
基于搜索机制混沌加密算法的密文特性分析

In order to solve the two major drawbacks of Baptista’s chaotic cryptosystem and its modified versions——excessive length of ciphertext and unbalance ratio of bit 0 to bit 1, this paper gives the lower bound of the expectation of the cipher N-to-plaintext ratio by calculating the entropy of the ciphertext. Then exploiting truncated equal length coding, the optimal truncated equal length coding and so on, this paper describes the relationship between coding and cipher N-to-plaintext ratio, and proposes the approximate formula to work out the ratio of bit 0 to bit 1. Numerical paradigms show that the optimal truncated equal length coding can efficiently shorten the length of ciphertext and obtain more uniform ratio of bit 0 to bit 1.